Use of a steel alloy for making tubes to produce compressed gas containers or for making formed structures in light weight steel construction

ABSTRACT

A method of using a steel alloy, the steel alloy having a composition containing in mass %, 0.09-0.13% C, 0.10-0.50% Si, 1.10-1.80% Mn, max. 0.02% P, max. 0.02% S, 1.00-2.00% Cr, 0.20-0.60% Mo, 0.02-0.06 Al, 0.10-0.25% V, the balance iron and incidental impurities, includes the steps of forming a tube from the steel alloy; air hardening the tube in the presence of inert gas, and incorporating the tube in the production of a compressed gas container. The steel composition may also be used as material for producing formed structures in light weight steel construction.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of prior filed copending PCT International application no. PCT/DE03/00394, filed Feb. 11, 2003, which designated the United States and on which priority is claimed under 35 U.S.C. §120, the disclosure of which is hereby incorporated by reference, and which claims the priority of German Patent Applications, Serial No. 102 06 612.4, filed Feb. 15, 2002, 102 21 487.5, filed May 15, 2002, and 102 21 486.7, filed May 15, 2002, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to use of a steel alloy for making tubes to produce compressed gas containers or for making formed structures in light weight steel construction.

Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.

Compressed gas containers are typically made of annealed carbon steel such as, e.g. St 52-3 or STE 460. When in addition to withstanding high compressive loads, there is a demand for a light weight construction of such containers, the use of liquid-hardened tempered steels has been proposed, such as SAE 1513. Production of compressed gas containers typically involve hot forming of tubular sections for shaping the neck and bottom of the containers. Then, the bottom center is closed. Another process involves production of compressed gas containers with welded-on bottom.

Regardless as to which way they are produced, compressed gas containers are subsequently tempered with oil or water, sand-blasted and then subjected to a pressure test. As a consequence of these procedures, internal voids of the compressed gas containers are scaled during oil tempering or water tempering. Thus, scale must be removed by a complicated sand blasting process. Another drawback is the application of oil cooling which entails health hazards. Also, the use of liquid hardening results in a relative wide range of hardness values in view of the geometric shape of the compressed gas containers (hollow bodies).

German Pat. No. 195 33 229 C1 discloses the use of a steel alloy as material for making pressure containers in the presence of inert gas. The steel alloy contains in particular expensive nickel as alloy element and can be air-tempered only after the pressure container has been made. After air hardening, it is hereby necessary to temper the pressure container in an additional process to realize the desired toughness. The tempering process is accompanied by a sooting of the surface which contradicts the need for a clean surface. Another drawback involves the high content of carbon which renders the steel alloy unsuitable when, for economical reasons, quick welding processes such as active-gas metal-arc welding with mixed gas (MAG) or laser beam welding, are demanded for making pressure containers, because of the risk of cracks.

Oftentimes, water-tempered fine-grained steel with a tensile strength Rm of 1,400 N/mm², yield point Rp0.2 of 1,000 N/mm², and stretch A5 of 8% is used in light weight steel construction, in particular automobile industry, for producing formed structures. This type of steel can be tempered only with water and only in the tool so that the overall production is cumbersome. Without tempering, this steel would lack a required strength, while tempering results in scaling that can be eliminated only by a comparably long pickling process which in turn leads to hydrogen embrittlement. Another drawback of this type of steel is a substantial loss in strength in heat impact zones that have been formed by welding seams, when tempered formed structures are joined together by welding. Although it may be conceivable to completely temper a formed structure that is composed of several single components, the finished formed structure is, however, warped or distorted. Loss in strength of such steel is also experienced during high temperature galvanizing, such as hot galvanizing. On the other hand, such galvanizing is, however, desired in particular in the production of formed structures in the automobile industry in order to realize a protection against corrosion at slight layer thickness.

Dual phase steel with a tensile strength Rm of 600 N/mm², yield point Rp0.2 of 400 N/mm², and stretch A5 of 20% has insufficient base strength. Increase in strength can be realized by higher transformation strain which, however, is undesired for many formed structures used in the automobile industry. This type of steel also experiences significant structural changes during high temperature galvanizing.

TRIP (short for “transformation induced plasticity”) steel (multiphase steel) having a tensile strength Rm of 700 N/mm², yield point Rp0.2 of 480 N/mm², and stretch A5 of 24% has also insufficient base strength. Although the strength could conceivably be increased by higher transformation strain, this course of action is, as stated above, undesired for many formed structures used in the automobile industry. Significant structural changes can also be encountered during high temperature galvanizing, and the strength profile of such steels is critical in the area of the welding seams and in the heat impact zones.

Special steel having a tensile strength Rm of 800 N/mm², yield point Rp0.2 of 370 N/mm², and stretch A5 of 53% results in high material costs while exhibiting an insufficient base strength. In addition, the strength of special steel can also only be increased by higher transformation strain which cannot always be attained in components of the automobile industry.

European Patent publications EP 1 143 022 A1, EP 1 041 167 A1 and EP 1 052 301 A1 disclose steel compositions which contain, in mass %, 0.09-0.12% C, 0.15-0.30% S, 1.10-1.60% Mn, max. 015% P, max. 0.011% S, 1.00-1.60% Cr, 0.30-0.60% Mo, 0.02-0.05% Al, 0.12-0.25% V, the balance iron and incidental impurities.

It would therefore be desirable and advantageous to provide an improved steel composition which obviates prior art shortcomings and is usable for a wide range of applications, including light weight steel construction, and which can be produced efficiently and easily with little material consumption, without requiring heat treatment after the manufacturing process and after welding.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a method of using a steel alloy, the steel alloy having a composition comprising, in mass %, 0.09-0.13% C, 0.10-0.50% Si, 1.10-1.80% Mn, max. 0.02% P, max. 0.02% S, 1.00-2.00% Cr, 0.20-0.60% Mo, 0.02-0.06 Al, 0.10-0.25% V, the balance iron and incidental impurities, includes the steps of forming a tube from the steel alloy, air hardening the tube in the presence of inert gas, and incorporating the tube in the production of a compressed gas container.

The present invention resolves prior art problems by using a steel composition for making compressed gas containers by applying the heat treatment step on the semi-finished tube during passage, thereby eliminating the need for tempering. As a result, the produced tube is clean and free of soot so that the manufacturing costs are reduced. The absence of the expensive alloy element nickel and the reduction in the content of chromium results in a cost-efficient steel composition. The steel alloy according to the present invention is suitable for application of quick welding processes such as MAG or laser beam welding to thereby further significantly reduce costs, when manufacturing compressed gas containers.

Welding tests, using MAGM welding, show a smaller hardness in the area of the welding seam and a hardness increase in the transition to the base material as well as heat impact zone. The highly desired increase in hardness in the heat impact zone is based on the precipitation hardening taking place there. As a result of the precipitation hardening, the tubes tear during a tensile test in the base material and do not tear in the area of the welding seam. Although the welding seam has a smaller hardness, this fact is balanced by the greater cross section. There is no risk of cracking. No further heat treatment (stress-free annealing) is required after welding.

The steel composition according to the present invention can have a tensile strength Rm of ≧950 N/mm², a yield point Rp0.2 of ≧700 N/mm², and a stretch A5 of ≧14%. In-house tests have shown that this steel composition has the required notch impact toughness of ≧35 Joule/cm² according to ISA V at −40° C.

In view of the attained high and even strength values and yield point as well as notch impact toughness solely by air hardening of the tubes, the cost saving is especially crucial when producing compressed gas containers on a large scale, e.g. as reaction containers for airbag systems, fire extinguisher containers or compressed gas bottles in beverage vending machines.

According to another feature of the present invention, the tube for making the compressed gas container is a seamless precision steel tube made from annealing a hot-rolled, seamless tubular blank until becoming soft, pickling the blank, phosphatizing the blank, soaping the blank, and drawing the blank to form the tube. Air hardening step is carried out in a continuous furnace at a temperature of 950° C.±15° C. in the presence of inert gas. Subsequently, the precision steel tube is straightened and subjected to an inspection, in particular ultrasonic inspection and/or eddy current testing. After cutting the tube to length, each sized tubular section is shaped to produce first the bottom and the neck of a compressed gas container and then shaped into the finished compressed gas container.

It is also possible to weld a bottom into each compressed gas container. Of course, instead of using a seamless tubular blank, it is also conceivable to use a longitudinally welded tubular blank.

According to another aspect of the present invention, a method of using a steel alloy, the steel alloy having a composition comprising, in mass %, 0.09-0.13% C, 0.10-0.50% Si, 1.10-1.80% Mn, max. 0.02% P, max. 0.02% S, 1.00-2.00% Cr, 0.20-0.60% Mo, 0.02-0.06% Al, 0.10-0.25% V, the balance iron and incidental impurities, includes the steps of forming a formed structure; and incorporating the formed structure in light weight steel construction. Examples of light weight constructions include crash-relevant vehicle parts such as crash boxes, rollover bars, side impact elements or column reinforcements. Light weight construction should have a certain strength while still showing certain elasticity (plastic deformation reserve), in order to be able to convert crash energy. The use of super high strength steel is unsuitable because of the limited capability of transformations and the absence of a needed plastic deformation in the event of a crash. A steel composition according to the present invention meets these criteria.

According to another feature of the present invention, the formed structure may be subjected to high temperature galvanizing. High temperature galvanizing even at a temperature of about 600° C. does hereby not adversely affect the strength of the air-tempered and air-hardened steel composition for making the formed structure for light weight steel construction. Indeed, the impact of high temperature galvanizing is positive in conjunction with air-hardened components because the temperature control transforms the components into the air-tempered state which is characterized by increased fatigue strength. In addition, application of high temperature galvanizing enables each formed structure to be provided with a slight layer thickness of zinc of about 20 μm to attain a sufficient long term protection against corrosion at slight zinc weights. Thus, the use of a steel composition according to the invention is especially applicable for light weight purposes as increasingly desired for formed structures, especially in the automobile industry.

The use of a steel composition according to the invention for making, in particular thin-walled formed structures in light weight steel construction, results in a higher base strength at acceptable stretch. Hardening in the heat impact zones ensures a reliable strength of the welded joints. A slight zinc weight is assured by small layer thickness while affording sufficient protection against corrosion through application of high temperature galvanizing. The application of a steel composition according to the invention is especially advantageous in the automobile industry for producing underbody parts such as struts or axle supports, because light weight and protection against corrosion at extreme conditions, as well as fatigue strength in the presence of dynamic loads are relevant criteria for these components. Using aluminum as alternative is not an option because of the significant added costs.

According to still another aspect of the present invention, a method of using a steel alloy, the steel alloy having a composition comprising, in mass %, 0.09-0.12% C, 0.15-0.30% Si, 1.45-1.60% Mn, max. 0.015% P, max. 0.011% S, 1.25-1.50% Cr, 0.40-0.60% Mo, 0.02-0.06% Al, 0.12-0.20% V, max. 0.20% Cu, max. 0.70% Ni, the balance iron and incidental impurities, includes the steps of forming a tube from the steel alloy; air hardening the tube in the presence of inert gas; and incorporating the tube in the production of a compressed gas container. This steel composition is especially suitable, when the wall thickness of the compressed gas container is at least 2 mm. In the event of a wall thickness of less than 2 mm, the steel alloy contains max. 0.2% of Ni.

According to yet another aspect of the present invention, a method of using a steel alloy, the steel alloy having a composition comprising, in mass %, 0.09-0.13% C, 0.15-0.30% Si, 1.10-1.60% Mn, max. 0.015% P, max. 0.011% S, 1.00-1.60% Cr, 0.30-0.60% Mo, 0.02-0.05% Al, 0.12-0.25% V, the balance iron and incidental impurities, includes the steps of forming a formed structure; air-hardening and tempering the formed structure to have a tensile strength of Rm of >850 N/mm², a yield point Rp0.2 of >700 N/mm², and a stretch A5 of >15%; and incorporating the formed structure in light weight steel construction.

According to another feature of the present invention, the formed structure may have a tensile strength of Rm of >950 N/mm², a yield point Rp0.2 of >700 N/mm², and a stretch A5 of >14%.

In general, the steel alloy can be provided through targeted adjustment of the alloy elements with intended properties such as:

-   -   a) peak adjustment at dynamic strength as a consequence of         stable bainitic base structure in air hardened and air tempered         state,     -   b) highest static strength and good stretching capability at the         same time,     -   c) retention of hardness for strength-stable heat impact zones         in case of welded joints.

Although the content of nickel in the steel alloy should be avoided or at least minimized in the light weight steel industry, a steel composition according to the present invention may, in general, contain traces of nickel up to a maximum of 0.20%. This content is the result of steel scrap when melting the steel alloy. The same is true for copper which is encountered as a result of scrap but limited to a maximum of 0.20% by weight.

BRIEF DESCRIPTION OF THE DRAWING

None

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

None

While the invention has been described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: 

1. A method of using a steel alloy, the steel alloy having a composition comprising, in mass %, C 0.09-0.13% Si 0.10-0.50% Mn 1.10-1.80% P maximum 0.02% S maximum 0.02% Cr 1.00-2.00% Mo 0.20-0.60% Al 0.02-0.06% V 0.10-0.25%

the balance iron and incidental impurities, said method comprising the steps of forming a tube from the steel alloy; air hardening the tube in the presence of inert gas; and incorporating the tube in the production of a compressed gas container.
 2. The method of claim 1, wherein the forming step includes annealing a hot-rolled seamless tubular blank until becoming soft, pickling the blank, phosphatizing the blank, soaping the blank, and drawing the blank to form the tube.
 3. The method of claim 2, wherein the air hardening step is carried out in a continuous furnace at a temperature of 950° C.±15° C.
 4. The method of claim 2, wherein the incorporating step includes straightening the tube, cutting the tube to length, and shaping the tube into the compressed gas container.
 5. A method of using a steel alloy, the steel alloy having a composition comprising, in mass %, C 0.09-0.13% Si 0.10-0.50% Mn 1.10-1.80% P maximum 0.02% S maximum 0.02% Cr 1.00-2.00% Mo 0.20-0.60% Al 0.02-0.06% V 0.10-0.25%

the balance iron and incidental impurities, said method comprising the steps of forming a formed structure; and incorporating the formed structure in light weight steel construction.
 6. The method of claim 5, further comprising the steps of air-hardening and tempering the formed structure to have a tensile strength of Rm of >850 N/mm², a yield point Rp0.2 of >700 N/mm², and a stretch A5 of >15%; and high temperature galvanizing the formed structure.
 7. The method of claim 6, wherein the formed structure has a tensile strength of Rm of >950 N/mm², a yield point Rp0.2 of >700 N/mm², and a stretch A5 of >14%.
 8. A method of using a steel alloy, the steel alloy having a composition comprising, in mass %, C 0.09-0.12% Si 0.15-0.30% Mn 1.45-1.60% P maximum 0.015% S maximum 0.011% Cr 1.25-1.50% Mo 0.40-0.60% Al 0.02-0.06% V 0.12-0.20% Cu maximum  0.20% Ni maximum  0.70%

the balance iron and incidental impurities, said method comprising the steps of forming a tube from the steel alloy; air hardening the tube in the presence of inert gas; and incorporating the tube in the production of a compressed gas container.
 9. The method of claim 8, wherein the steel alloy contains a maximum of 0.2% of Ni.
 10. A method of using a steel alloy, the steel alloy having a composition comprising, in mass %, C 0.09-0.13% Si 0.15-0.30% Mn 1.10-1.60% P maximum 0.015% S maximum 0.011% Cr 1.00-1.60% Mo 0.30-0.60% Al 0.02-0.05% V 0.12-0.25%

the balance iron and incidental impurities, said method comprising the steps of forming a formed structure; air-hardening and tempering the formed structure to have in the air-tempered state a tensile strength of Rm of >850 N/mm², a yield point Rp0.2 of >700 N/mm², and a stretch A5 of >15%; and incorporating the formed structure in light weight steel construction.
 11. The method of claim 10, wherein in the air-hardened state the formed structure has a tensile strength of Rm of >950 N/mm², a yield point Rp0.2 of >700 N/mm², and a stretch A5 of >14%.
 12. The method of claim 10, and further comprising the step of high temperature galvanizing the formed structure.
 13. A compressed gas container, comprising a tube, said tube being formed from a steel alloy comprising, in mass %, C 0.09-0.13% Si 0.10-0.50% Mn 1.10-1.80% P maximum 0.02% S maximum 0.02% Cr 1.00-2.00% Mo 0.20-0.60% Al 0.02-0.06% V 0.10-0.25%

the balance iron and incidental impurities.
 14. A compressed gas container, comprising a tube, said tube being formed from a steel alloy comprising, in mass %, C 0.09-0.12% Si 0.15-0.30% Mn 1.45-1.60% P maximum 0.015% S maximum 0.011% Cr 1.25-1.50% Mo 0.40-0.60% Al 0.02-0.06% V 0.12-0.20% Co maximum  0.20% Ni maximum  0.70%

the balance iron and incidental impurities.
 15. A formed structure for use in light weight steel construction, made from a steel alloy comprising, in mass %, C 0.09-0.13% Si 0.10-0.50% Mn 1.10-1.80% P maximum 0.02% S maximum 0.02% Cr 1.00-2.00% Mo 0.20-0.60% Al 0.02-0.06% V 0.10-0.25%

the balance iron and incidental impurities.
 16. A formed structure for use in light weight steel construction, made from a steel alloy comprising, in mass %, C 0.09-0.13% Si 0.15-0.30% Mn 1.10-1.60% P maximum 0.015% S maximum 0.011% Cr 1.00-1.60% Mo 0.30-0.60% Al 0.02-0.05% V 0.12-0.25%

the balance iron and incidental impurities, and having in air-tempered state a tensile strength of Rm of >850 N/mm², a yield point Rp0.2 of >700 N/mm², and a stretch A5 of >15%
 17. The formed structure of claim 16, having in air-hardened state a tensile strength of Rm of >950 N/mm², a yield point Rp0.2 of >700 N/mm², and a stretch A5 of >14%.
 18. A steel alloy, essentially consisting of, in mass %, C 0.09-0.13% Si 0.10-0.50% Mn 1.10-1.80% P maximum 0.02% S maximum 0.02% Cr 1.00-2.00% Mo 0.20-0.60% Al 0.02-0.06% V 0.10-0.25%

the balance iron and incidental impurities.
 19. A steel alloy, essentially consisting of, in mass %, C 0.09-0.12% Si 0.15-0.30% Mn 1.45-1.60% P maximum 0.015% S maximum 0.011% Cr 1.25-1.50% Mo 0.40-0.60% Al 0.02-0.06% V 0.12-0.20% Co maximum  0.20% Ni maximum  0.70%

the balance iron and incidental impurities.
 20. The steel alloy of claim 19, containing a maximum of 0.2% of Ni.
 21. A steel alloy, essentially consisting of, in mass %, C 0.09-0.13% Si 0.15-0.30% Mn 1.10-1.60% P maximum 0.015% S maximum 0.011% Cr 1.00-1.60% Mo 0.30-0.60% Al 0.02-0.05% V 0.12-0.25%

the balance iron and incidental impurities. 